Cathode



Jan. 16, 1945.

R. J. BONDLEY 2,367,332

CATHODE Filed June 26, 1942 Fl g. l.

/// O, I I /Zf 20 /0 m c m w r r 2 22x 22 Ff 2 /7 j 3 4i i Inventor:Ralph J. Bondle y;

His Attorney.

- Patented Jan. 16, 1945 CATHODE Ralph J. Bondlcy, Scotia, N. Y.,assignor to General Electric Company, a corporation of New YorkApplication June 26, .1942, Serial No. 448,541

Claims.

The present invention relates to an improved form of cathode especiallyadapted for use in ultra high frequency discharge tubes.

Certain high .frequency tubes of a type described in E. D. McArthurPatent No. 2,353,743, dated July 18, 1944, employ a cathode structurecomprising an elongated hollow cylinder having an emissive part whichforms an end wall for the cylinder. This construction has numerousadvantages as used in a high frequency system. but presents certaindifliculties in maintaining the emissive part at an emittingtemperature, these difficulties being largely attributable to heat lossfrom the emissive part to the structure by which it is supported.

It is an object of the present invention to provide an improved cathodehaving the advantages of the structure referred to in the foregoing butcharacterized by a low rate of heat loss from the emissive component ofthe cathode. In the attainment of this object an important feature ofthe invention comprises the use of a piece of thin metal foil asaprincipal element of the structure by which the emissive cathode part issupported. As will appear more fully in the following, the low thermalconductivity of such an element tends to minimize conductive heat lossto the supporting parts while at the same time making it possible toprovide a continuous imperforate structure with the advantages whichaccrue from such a structure when used in a high frequency system.

The features of the invention desired to be protected herein are pointedout with particularity in the appended claims. The invention itself,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the drawing in which Fig. 1 is a longitudinal sectional view of adischarge tube suitably embodying the invention and Figs. 2' and 3 aredetail views of certain parts of the cathode structure shown in Fig. 1.

Referring particularly to Fig. 1, there is shown a high frequencydischarge tube formed of a series of three similar metal disks III, IIand I2 which are insulatingly separated by glass cylinders l3 and I4sealed between them. The upper disk It] supports a cylindrical anodestructure which extends centrally through the disk and which terminatesexternally in a threaded portion l9 adapted for connection to a. currentsupply terminal. The anode has a central bore 20 which connects with themain discharge space through openings 22 and through which evacuation ofthe discharge envelope can be carried out. After evacuation, theenvelope is sealed by means of a body of glass 24 fused to the extremityof a metal tubulation 25 which is brazed into the anode bore 2|].

Below the anode and at a distance of a few mils from it there isprovided a grid which comprises a series of conductive rods 30 placed atregularly spaced intervals across a circular opening formed centrally inthe disk H, the outer periphery of the disk thus providing a terminalfor the grid.

The cathode of the tube comprises an emissive part 33 which is in theform of a disk of refractory material, such as nickel or molybdenum, andwhich is provided peripherally with a depending flange 33'. This diskmay be coated on its upper surface with an emission enhancing material,such as alkaline earth oxide. A support or mount for the disk isprovided by a metal cylinder 35 which is supported centrally withrespect to the partlZ, being secured to it by being welded or brazed toa heavy metal sleeve 36 which is set into an opening formed in the partreferred to. The connection between the supporting cylinder 35 and thedisk 33 comprises a sleeve 38 which is constituted of thin metal foil,preferably a foil having a thickness between a fraction of a mil (e. g.0.3 mil) and a few (3 or 4) mils. The function and utility of thissleeve will be explained in detail at a later point.

Lil

Within the hollow cathode structure formed by the combination of thecylinder 35 and the foil sleeve 38 there is provided a radiation heatercomprising a multiply bent filament 40 (see Fig. 2). This is arranged inclose proximity to the emissive part 33 and during the operation of thedevice serves to maintain the latter part at an elevated temperature.Heating current is supplied to the filament 40 through lead-in wires 4|which extend downwardly through the.

disk I 2, being insulatingly supported with respect to the disk by meansof a body of glass 43 fused into a flanged eyelet 44.

In orderto assure efficient operation of the cathode as a whole it isdesirable to prevent the heat generated by the filament 40 from beingdissipated by radiation or conduction to the non-emissive parts of thecathode structure. Radiation in unwanted directions is minimized by theuse of a shield 46 which comprises a metal plate or reflector supportedby one of the lead-in conductors 4| at a location just below the planeof the filamentary heater 40. Loss of heat by conduction, on the otherhand, is reduced to a low value due to the high resistance presented bythe foil sleeve 38 to heat flow along it. In other words, the very smallcross-section of the heat transfer path provided through and along thefoil makes it certain that heat losses by this means will be extremelysmall.

From the standpoint of using the tube in a high frequency system, thecathode structure has the advantage that the heater 40 and itsassociated leads are wholly enclosed by a conductive structure so thatno coupling exists between these parts and the high frequency fieldsdeveloped in the space near the outer electrode surfaces. Morover, thefoil 38 provides a wholly symmetrical conductive path for the flow ofhigh frequency currents to and from the emissive disk 33 to the cathodeterminal provided by the disk l2, this being a highly advantageouscircumstance from the standpoint of assuring minimum inductance for thecathode circuit.

In applying the foil 38 to the cathode structure a, sleeve ofappropriate dimensions may first be formed by wrapping foil ribbon abouta cylindrical mandrel of the correct diameter and then welding theoverlapping edges. The cylindrical sleeve thus provided is nextmanipulated into the proper relationship with the emissive part 33 andthe supporting cylinder 35 and is spot-welded at its extremities tothese parts to form an assembly such as that illustrated.

The foil 38 may be formed of any reasonably heat-resistant material,such as nickel or molybdenum. In a, particular case it has been foundadvantageous to use a foil which is constituted of nickel-iron-cobaltalloy (fernico) because of the extremely low heat conductivity of thismetal. Due to its cylindrical form the foil part is characterized by amuch higher degree of mechanical strength than its physical dimensionswould indicate. Even when a very thin foil is employed, there is noparticular tendency for the foil to sag or to permit displacement of theemissive part 33.

While the invention has been described by reference to a particularembodiment of it, it will Letters Patent of the United be understoodthat numerous modifications may be made by those skilled in'the artwithout departing from the invention. I therefore aim in the appendedclaims to cover all such equivalent variations as come within the truespirit and scope of the foregoing disclosure. I

What I claim as new and desire to secure by States, is:

1. A cathode comprising an emissive part, a support for said cathode anda. piece of me 1 foil providing a. connection between said emissive partand said support for minimizing heat transfer from said emissive part tosaid support.

2. A cathode including an emissive part desired to be maintained at anelevated temperature, a support for said cathode, and a metallicconnection of low thermal conductivity between said part and saidsupport, said connection comprising a piece of metal foil having athickness between a, fraction of a mil and a few mils.

3. A cathode including an emissive disk desired to be maintained at anelevated temperature and means for supplying current symmetrically tothe disk while minimizing conductive heat loss from the disk, said meanscomprising metal foil attached to the periphery of the disk.

4.-A cathode including an emissive part, a hollow mounting structure forsaid part, a heater within said structure for maintaining said part at atemperature of thermionic emission, and means for minimizing conductiveheat loss from said part, said means comprising a sleeve of thin metalfoil forming an element of said mounting structure.

5. A cathode including an emissive disk, a filamentary heater positionedin efiective heat exchanging relation to said disk, a. metal cylinderproviding a support for said disk, and a sleeve of thin metal foilproviding a connection between an extremity of said cylinder and theperipheral edge of said disk, said cylinder and sleeve in combinationwith said disk forming an enclosure for said heater and said sleeveserving to minimize conductive heat transfer between said disk and saidcylinder.

- RALPH J. BONDLEY

